The Global Positioning system (GPS) is a time-synchronized space-based satellite system that broadcasts spread spectrum codes from a nominal constellation of 24 earth-orbiting satellites. GPS uses Code Division Multiple Access (CDMA) to simultaneously broadcast multiple codes on each GPS frequency. Since the standard GPS constellation consists of 24 satellites, 24 codes are normally broadcast simultaneously on each GPS frequency.
Because each satellite broadcasts a unique code, and because the codes have poor cross-correlation properties, it is relatively easy to use matched replica correlation processing to extract any particular code from the rest. In matched replica processing, a replica of a code sequence is compared with the received signal. The replica will correlate with the received signal only if the received signal contains the sequence of codes that match the replica and only when the replica is correctly aligned with the code sequence.
The GPS satellite constellation is continuously monitored, with ephemeris and clock corrections broadcast so that receiving equipment can locate GPS satellites at any time and make corrections necessary to account for errors in each satellite's clock. The distance from the satellite to the receiver can be determined from the location of a satellite, the time that the satellite's coded message was transmitted, and the nominal time the message was received. If codes are received from four or more GPS satellites, then the receiver's position and the error in the receiver's clock can be estimated. It is necessary to ensure that the receiver's clock is synchronized with the satellite constellation's clock in order to accurately measure the elapsed time between a satellite's code sequence transmission and reception of that code sequence by a GPS receiver.
GPS can provide highly accurate position estimates. However, the GPS signal frequency is approximately 1.5 GHz and its power level at the receiving antenna is -160 dBW. The high frequency and low received power of the GPS signals restrict the use of GPS to locations where the receiver's antenna has a clear, line-of-sight view of the requisite number of satellites. The requisite number of satellites is normally four, but can be less if some form of aiding is used. For example, if altitude aiding is used, it may be possible to obtain a GPS solution with three satellites. This line-of-sight restriction degrades GPS performance in buildings, in vehicles, under foliage, in areas with steep terrain where the horizon is blocked by mountains or high buildings, or other places where the GPS antenna does not have an unobstructed view of the sky.